EXTRA CREDIT OPPORTUNITY: Due end of day, Thursday, Dec. 14

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Shavonne Terry
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1 EXRA CREDI OPPORUNIY: Due end of day, hursday, Dec. 4 his extra credit set of questions is an opportunity to improve your test scores (including an insurance policy for your final exam grade). here are 7 multiple choice questions in the following pages, each worth point. he points you receive for all questions answered correctly will be added to your total test score (including two midterms and the final). For example, if you received scores of 60 on midterm, 58 on midterm, and 70 on the final exam and answered all 7 extra credit questions correctly, your total point score for the exam component of the class would be: = 5, for a test average of 7.7 (this would be an improvement over your total test score without extra credit = 88, average = 6.7). he application of the extra credit will have a ceiling so that no one gets an average of over 00 for the three tests. Of course, if you are satisfied with your test scores you do not have to do any of these problems. Instructions: the questions are open book and notes, but they must be solved by you on your own. Show your work that justifies your answer. Questions involve some computation or explanation and you will not receive credit for an answer if you do not show your work or reasoning, i.e., there is no credit for guessing. If you have questions, see Prof. Hernandez or Silverstein. Please sign the honor code statement below before turning in the test, indicating that you have not received any unauthorized assistance. Do not worry if your answer is not exact. Select and circle the closest of the choices only one answer is correct for each question. Staple your worksheets to the exam, with the problems clearly numbered, when you turn it in. Name (print) I have read, understand, and agree to abide by the University of Colorado honor code in this test context: I have neither given nor received unauthorized assistance during this examination. Signed: Score: /7 points possible

2 . A vessel with a volume of m 3 contains liquid water and water vapor in equilibrium at 600 kpa. he liquid phase has a mass of kg. he mass of water vapor is: (a) 0.99 kg (b).6 kg (c).9 kg (d) 3. kg. he general equation for conservation of energy ( st Law) involves the following 5 terms: I. accumulation of energy in the control volume II. net energy transfer by work III. net energy transfer by heat IV. transfer of energy in by mass flow V. transfer of energy out by mass flow Using the standard sign conventions, which of the relations below is the correct arrangement of these terms that satisfies the st Law of hermodynamics? (a) I = II + III + IV V (b) I = II + III + IV + V (c) I = II + III + IV V (d) I = II III IV + V 3. In a reversible process, the state of a system changes from state to state as shown on the P- v diagram. What does the shaded area of the diagram represent? P v (a) free energy change (b) heat transfer (c) enthalpy change (d) boundary work

3 4. How much power is required to isothermally compress 3 m 3 /min of air from 0.5 kpa to 88.5 kpa? (a) 64 kw (b) 8 kw (c) 9 kw (d) 98 kw 5. Gas is in a closed piston-cylinder system. he gas is heated and expands from a volume of 0.04 m 3 to 0.0 m 3. he pressure varies such that the product P*v is constant, and the initial pressure is 00 kpa. What is the work done by the system? (a) 6.8 kj (b) 7.3 kj (c) 0 kj (d) kj 6. Steam flows into a turbine with the characteristics below at a rate of 0 kg/s and 0 kw of heat are lost from the turbine. Property inlet exit Pressure (MPa).0 0. emperature ( o C) Quality -- Neglecting potential and kinetic energy changes, what is the power output from the turbine? (a) 4,000 kw (b) 4,400 kw (c) 4,600 kw (d) 5,000 kw 7. How does an adiabatic process compare to an isentropic process? (a) adiabatic: heat transfer = 0; isentropic: heat transfer 0 (b) adiabatic: heat transfer 0; isentropic: heat transfer = 0 (c) adiabatic: reversible; isentropic: not reversible (d) both: heat transfer = 0; isentropic: reversible 8. An isobaric steam generating process starts with saturated liquid at 43 kpa ( s = 0 o C). he change in entropy during the steam generating process is EQUAL to the initial entropy (e.g., Δs = s ). Not all the liquid is vaporized. What is the change in enthalpy (Δh) during the process? (a) 300 kj (b) 46.3 kj/kg (c) 46.3 kj/kg (d) kj/kg 3

4 9. A piston-cylinder device contains saturated water vapor at 0 o C. he vapor is compressed in a reversible adiabatic process until the pressure is.6 MPa. What is the work done in the process? (a) 640 kj/kg (b) 50 kj/kg (c) 430 kj/kg (d) 330 kj/kg 0. For which type of process is the equation dq = ds valid? (a) irreversible (b) isothermal (c) reversible (d) isobaric. In the P-v diagram shown, heat addition to an ideal gas occurs in the process between states and. If c v for the gas is constant and = kj/kg-k. What is the entropy produced during this process. P : 3,78 kpa, 534K :,48 kpa, 600K (a) 0.4 kj/kg-k (b) 0.3 kj/kg-k (c) 0.3 kj/kg-k (d) 0.4 kj/kg-k v. An ideal reversible Carnot cycle is represented on the -S diagram below. he efficiency of the cycle is represented by which of the following ratios of areas? H L S 3 6 (a) (b) (c) (d)

5 3. In the Carnot cycle shown below, the net amount of heat put into the system is equal to the net amount of work done by the system. However, it cannot be stated that the heat put into the system between states and is equal to the work done between states and. What is the reason for this? 4 3 S (a) the process is adiabatic (b) the process is not adiabatic (c) the nd law states that the amount of energy put into the system is equal to the amount of energy taken out of the system (d) from the st law: dq = du + dw. It cannot be assumed that du = 0, so dq dw. 4. For a heat engine operating between two temperatures where ( > ), what is the maximum efficiency attainable? (a) (b) (c) (d) k 5. Steam expands from MPa and 00 o C to a mixture, x = 0.83 and 40 o C. What is the change in entropy of the steam? (a) 0.35 kj/kg-k (b) 0 kj/kg-k (c) 0.6 kj/kg-k (d) 0.73 kj/kg-k 6. Refrigerant (R-34a) is throttled in an adiabatic valve from h = 480 kj/kg and P = 0. MPa to a pressure of 0.06 MPa. What is the final enthalpy (h )? (a) 00 kj/kg (b) 300 kj/kg (c) 480 kj/kg (d) 560 kj/kg 5

6 7. he enthalpy of R-34a is reduced from 440 kj/kg to 300 kj/kg in a condenser maintained at 0.8 MPa (constant P). What is the final quality of the R-34a? (a) 3% (b) 37% (c) 63% (d) 7% 8. Steam enters an adiabatic steady-state nozzle at MPa, 50 o C, and 30 m/s. At one point in the nozzle the enthalpy has dropped to 40 kj/kg less than the inlet value. Determine the velocity of the steam at that point. (a) 3 m/s (b) 0 m/s (c) 50 m/s (d) 80 m/s 9. Air is compressed from 00 kpa and 40 o C to,500 kpa and 30 o C in a steady-flow process. During the compression process, each kilogram of air loses 90 kj as heat to the surroundings. Air is discharged at the rate of 0 m3/min. he required power input for the compressor is (a) 6 kw (b) 80 kw (c) 95 kw (d) 39 kw 0. Which of the following statements is the best interpretation of the st Law of hermodynamics for a closed system? (a) he mass within a closed control volume does not change (b) he net energy crossing the system boundary is the change in energy inside the system (c) he change in total energy of the system is equal to the rate of work performed (d) All real processes tend toward increased entropy in the universe -3. An ideal gas goes though a cycle consisting of three processes: A B: isothermal compression B C: isochoric cooling where C, P C =.4 bars and V C = 0.08 m 3 C A: isobaric expansion with net work = 0.5 kj. What is the volume at state A? (a) 0.07 m 3 (b) 0.0 m 3 (c) 0.9 m 3 (d) 0.4 m 3. What is the work in the process A B? (a) 9 kj (b) 3 kj (c) 0 kj (d) 5.3 kj 6

7 3. What is the net work of the cycle? (a) kj (b) 8.3 kj 6.5 kj (d) 4.8 kj 4. A steam pipe operating at steady-state receives 30 kg/min steam with an enthalpy of,900 kj/kg. If the enthalpy of the steam at the pipe exit equals,600 kj/kg, what is the rate of heat transfer from the pipe? (Neglect kinetic energy) (a) - 40 kj/min (b) kj/min (c) -,300 kj/min (d) - 39,000 kj/min 5. A flow of m 3 /min liquid oil is to be heated from 0 to 00 o C (stays as liquid) in a heat exchanger. he heating fluid is steam which enters the heat exchanger at 50 kpa and x = 0.9. he heat exchanger loses 5% of the heat transferred from the steam to the oil to the surroundings. he steam leaves the heat exchanger as saturated liquid. What is the mass flow rate of steam (kg/hr) required to heat the oil? (he density of the oil, ρ = 880 kg/m 3 and the specific heat =.00 kj/kg-k.) (a),00 kg/hr (b) 4,600 kg/hr (c) 8,600 kg/hr (d) 9,000 kg/hr 6. A refrigeration cycle has a coefficient of performance that is 80% of the Carnot refrigerator operating between temperature reservoirs at 33K and 68K. he rate of heat transfer from the low temperature reservoir is 3 kw. What is the required power input to the real refrigerator (not the Carnot refrigerator)? (a) 0.53 kw (b) 0.6 kw (c) 0.77 kw (d) 0.89 kw 7. An inventor claims that an engine produces 30 kw with a fuel consumption of 0 kg/hr. he energy content of the fuel transferred to the engine is 40,000 kj/kg when burned at 500 o C. Heat is rejected from the engine at 50 o C. Which laws of thermodynamics are violated by this claim? (a) First Law only (b) Second Law only (c) Both First and Second Laws (d) Neither First nor Second Laws 7

Chapters 5, 6, and 7. Use T 0 = 20 C and p 0 = 100 kpa and constant specific heats unless otherwise noted. Note also that 1 bar = 100 kpa.

Chapters 5, 6, and 7. Use T 0 = 20 C and p 0 = 100 kpa and constant specific heats unless otherwise noted. Note also that 1 bar = 100 kpa. Chapters 5, 6, and 7 Use T 0 = 20 C and p 0 = 100 kpa and constant specific heats unless otherwise noted. Note also that 1 bar = 100 kpa. 5-1. Steam enters a steady-flow device at 16 MPa and 560 C with